Works fine for me, so I'm guessing you're not looking for files in the right places:

Code:

$ mkdir dir
$ cd dir
$ touch IIFTa
$ touch WWFT
$ TD=2011-11-11
$ find . -type f -name 'IIFT*' -size 0 -exec mv WWFT WWFT$TD \;
$ ls
IIFTz  WWFT2011-11-11
$

Note that yours excludes the current directory. I did find . and you did find dir/

If this file is buried several levels deep, you'll probably need find, but if it's always in the same place, there might be an easier way to do it in shell. What's your system, what's your shell?